Field of the Invention
The present invention relates to the use of a methylated amorphous silicon alloy as the active material of an anode for a Li-ion battery. It relates to the negative electrodes (anodes) manufactured using this material, as well as the lithium storage batteries whose anode comprises this material. The invention has also for object a method for manufacturing such electrodes.
The material of the invention differs from those already known for manufacturing anodes for lithium-ion storage batteries by its high charge/discharge capacity, its high coulombian efficiency, the improvement of the cycling properties (a greater number of cycles is possible), its high charge-discharge speed, its easiness of implementation.
Description of the Related Art
The large increase of the use of portable electronic devices requires storage batteries having higher and higher energetic capacities. In particular, this concerns the portable communication devices, the portable electronic devices, notably the computers. The electric or hybrid vehicles are another important application for high capacity storage batteries.
Such devices widely use lithium-ion batteries. However, the present performances of this type of batteries are insufficient. The anode is an important component of a lithium battery. The role of the anode consists in lithium insertion or incorporation during the cycle of charge of the battery, and lithium release when the battery is discharged. In many cases, the lithium insertion and release may lead to volume changes that cause physical perturbations in the electrochemically active material of the anode and thus compromise the integrity thereof. This loss of integrity leads to a decrease of performance of the battery with the repetition of the charge and discharge cycles. Hence, it is expected that the stability and the performances of the battery will be improved if this degradation of the electrode materials can be avoided or reduced.
The lithium-ion storage batteries presently available in the market comprise a carbon-based anode and a cathode based on doped LiNiO2, LiCoO2, LiMnO2 or doped LiMn2O4. Such storage batteries have limited mass and volume energies. The carbon-based anode of these batteries has a rather long life span, but a limited reversible capacity. The reversible capacity is defined as the quantity of electricity generated during the discharge by the reversible di-insertion of the lithium atoms out of the anode. Hence, the energy delivered by a lithium storage battery is limited by its anode.
Silicon has been known for long as a very promising material for the Li-ion batteries due to its capacity to accept insertion of great quantities of lithium, a capacity that is twelve times higher than that of the graphite used in the commercial batteries. The use thereof has been greatly limited until now by the very large swelling of the material in the charging state (about 300%). Solutions to this swelling problem have been proposed. They require the use of silicon in nanometric form (layers of nanometric thickness, powder with grains of nanometric size, nanowires or nanotubes).
If these materials have a high theoretical mass capacity (2000 to 4000 mAh/g), their strength to cycling remains very bad, because the very large volume variations caused by the lithium insertion/de-insertion cycles harm the integrity of these highly divided materials.
Moreover, the stoichiometric SiC alloy does not accept lithium insertion. On the other hand, the use of carbon and silicon in an heterogeneous mixture (Si powder surrounded by carbon, binders, etc. . . . ) allows improving the cycling properties of the silicon, but without fully solving the problems linked to the swelling due to the lithium.
The hydrogenated amorphous silicon (a-Si:H) is a material in which the silicon atoms adopt a tetrahedral local environment (corresponding to a hybridization state referred to as sp3), as in the crystalline silicon, but whose arrangement has no order over a long distance, contrary to the crystalline silicon. The material, which may be advantageously obtained as a thin layer by plasma-enhanced decomposition of silane (SiH4) (according to a plasma-enhanced chemical vapor deposition technique, known as PECVD), contains a high quantity of hydrogen, a part of which is mobile within the material and a part of which is engaged in links with silicon atoms. The presence of hydrogen in the material allows providing it with semi-conductor properties, by limiting the density of the electronic states in its forbidden band, which allows in particular doping it and providing it with a conduction through the electrons of its conduction band (N-type material) or through those of its valence band (P-type material). The hydrogenated amorphous silicon is a material whose use as thin layers has been contemplated for the lithium batteries. Although it does not suffer from the drawback of having to undergo a transition from the crystalline state to the amorphous state during the first charge/discharge cycle, the materiel proves to have performances, in terms of cycling, which are limited to about twenty cycles (see for example H. Jung et al., Journal of Power Sources 115 (2003) 346-351).
W. J. Zhang, J. Power Sources 196 (2011) 13-24 proposes a review of the studies about the electrochemical properties of the new materials for Li-ion battery anodes.
Several authors have proposed Si—C composites that are stable or pseudo-stable under cycling and that have mass capacities of 500 to 900 mAh/g.
A composite is defined as a material formed of several components to obtain particular mechanical properties. However, the composite synthesis is accompanied by the formation of silicon carbide, which harms the electrochemical performances of these composites.
In particular, D. Larcher et al., Solid State Ionics 12, 71 (1999); D. Larcher et al., Electrochim. Acta 44, 406>9 (1999) have described SiC composites prepared by pyrolysis under argon of pitch/polysilane blends. This method leads to Si-rich nanoparticles included in a disordered carbon matrix.
Other composites have been described by J. Yang et al., Electrochem. Solid State Lett. 6, 154 (2003); J. Saint et al., Adv. Funct. Mater. 17, 1765 (2007); FR2885734; EP1722429. They are SiC composites obtained by pyrolysis at 900° C. of intimate mixtures of Si/PVC under N2. The reversible capacity is of about 1000 mAh/g during 20 cycles. The irreversible capacity is of about ⅓ of the reversible capacity. The authors have observed that the carbon coating exerts a force of compression on the silicon inclusions, which is necessary for a good cyclability. A grinding, even moderated, of the material destroys this effect.
WO2007/053704A2 describes a new material for negative electrodes of lithium batteries. This material is consisted of an active material subjected to volume variations and of a metal or metal-oxide based buffer material improving the cyclability. But, as with the composites, the swelling of the active material requires a demanding arrangement between the active material and the buffer material, and this arrangement proves difficult to implement for a high-capacity electrode.
Besides, A. M. Wilson, J. R. Dahn, J. Electrochem. Soc. 142, 326 (1995) have described the preparation of silicon nanoparticles maintained in a graphite matrix, by CVD of a silane/methane mixture. However, a lack of reproducibility of this method has been observed and the cost of these materials is very high.
JP2005-235397 describes anodes for lithium-ion batteries based on amorphous silicon and at least one impurity chosen among carbon, nitrogen, oxygen, argon and fluorine. However, it is observed that the tests of cyclability are limited to a very low number of charge/discharge cycles (five) and that, for a concentration of carbon exceeding 2%, the inventors didn't even manage to reach this number. The cyclability of the described materials hence appears to be very low. As regards the carbon materials, it can be noted that the conditions of preparation by PECVD require a high power for exciting the plasma, which very likely leads to the formation of graphite carbon.